1. Field of the Invention
The invention relates to a radiation therapy unit with a beam of rays propagating from a focal point along a beam axis and a radiator head arranged on the beam axis, having the following features:
(a) the radiator head comprises a double-focus multi-leaf collimator; PA1 (b) the multi-leaf collimator exhibits a plurality of adjacently arranged diaphragm plates which in each case have two side faces, two front faces and an inside and an outside face; PA1 (c) means are provided for displacing each individual diaphragm plate. PA1 each side face of each diaphragm plate forms a part of a surface area of a cone, all such cones having both a common cone axis which extends perpendicularly to the beam axis through the focal point and a common cone point which coincides with the focal point, and PA1 means are provided for guiding the diaphragm plates so that each diaphragm plate performs a pure rotation about the cone axis during its displacement.
2. Discussion of Background
Radiation therapy units are used in medicine for therapeutically treating tumors by means of high-energy photons or electrons. In this connection, it is of importance that the beam of rays generated by the unit has accurately defined characteristics with respect to field limiting. In this respect, radiation therapy units having multi-leaf collimators of the type initially mentioned are particularly suitable.
Such a unit is known, for example, from U.S. Pat. No. 4,672,212. Instead of a conventional pair of collimator blocks, a multi-leaf collimator is used there. In order to avoid unwanted half shadows at the edge of the radiation field, the multi-leaf collimator is focussed twice. For this purpose, the individual diaphragm plates are segments of a circular ring and have a cross section of the shape of an equal-sided trapezoid. The multi-leaf collimator composed of the diaphragm plate is thus circularly bent in two directions which are perpendicular to one another and thus imitates a part of a spherical shell (only approximately, however).
Each diaphragm plate is provided at its rear end with a rod pointing upwards. The diaphragm plate is advanced and retracted on this rod by means of a motor.
The problem of this multi-leaf collimator lies in the fact that an unwanted leakage radiation occurs between the individual diaphragm plates. A further disadvantage lies in the complicated drive arrangement. This is because the diaphragm plates move on curved paths which change from plate to plate. The type of coupling proposed between motor and diaphragm plate leads to a non-linear relationship between plate advance and motor speed.
The published European patent application EP-0,259,989 A1 describes quite a different multi-leaf collimator. This is used in a conventional radiation therapy unit in addition to the two pairs of collimator blocks, for shielding sensitive organs of the patient. The multi-leaf collimator consists of a plurality of laterally adjoining diaphragm plates. The essentially rectangular plates are slightly rounded on their front and have straight-line guide slots at the side faces. They run along straight tracks and are driven by associated motors by means of flexible cables.
Although this multi-leaf collimator has a linear drive arrangement, it can only be used in conjunction with the conventional collimator blocks due to the lack of double focussing.